This application claims the priority of 011 27 990.8, filed Nov. 24, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for High Pulse Repitition Frequency (HPRF) radar measurement of target range and Doppler frequency, such as may be used, for example, in an airborne radar system.
The use of a well known radar waveform with two slightly different transmit frequencies is often referred to in the literature as a frequency shift keying (FSK) technique. Using such a technique, target range and velocity can be measured unambiguously even in multi target situations. FIG. 1 shows the two carrier frequencies f1 and f2 as a function of time. Each frequency represented by a horizontal line corresponds to a coherent processing interval (CPI), and includes a pulse sequence of N-2048 pulses each.
In each CPI of length T, a received pulsed signal is directly down converted with the individual instantaneous transmit frequency, and shows as a single information the Doppler frequencies of the observed targets. The two base band signals are Fourier transformed, and target line left out will be observed in two adjacent CPI""s in the same Doppler bin, but with different phase information. For a simultaneous range measurement, these two phases xcfx861, xcfx862 in the same Doppler bin are considered and the phase difference xcex94xcfx86=xcfx862xe2x88x92xcfx861 is calculated. The latter is directly related to the target range R in accordance with the following equation:                               Δ          ⁢                      xe2x80x83                    ⁢          φ                =                  2          ⁢                      π            ·            Δ                    ⁢                      xe2x80x83                    ⁢                      f            ·                                          2                ⁢                R                            c                                                          Eq        .                  xe2x80x83                ⁢                  (          1          )                    
The range measurement is unambiguous if xcex94xcfx86 less than 2xcfx80. In order to measure target ranges up to Rmax=150 km, the frequency shift in the FSK wave form must be:             Δ      ⁢              xe2x80x83            ⁢      f        ≤          c              2        ⁢        R              =      1    ⁢          xe2x80x83        ⁢          kHz      .      
The basic idea is applicable to the extent that the assumptions of a purely stationary observation are fulfilled. But each target maneuver inside the measurement time 2T will have a large influence on the phase measurement in the adjacent CPI""s, and will cause large range errors in the range estimation procedure in accordance with Eq. 1.
One object of the present invention is to provide a method for simultaneously and unambiguously measuring target range and target Doppler frequency for highly mobile targets, even in multiple target situations.
This and other objects and advantages are achieved by the present invention, in which a transmit oscillator generates a single unmodulated frequency. The HPRF pulse sequence is split into two interleaved parts, which are distinguished and modulated with a digital phase shift keying (PSK) modulation scheme as follows:
The pulses of the first pulse sequence have the same phase state or alternatively have a linearly increasing phase value with a fixed phase difference xcfx861n from pulse to pulse; and
The pulses of the second pulse sequence have a linearly increasing phase value with a fixed phase difference xcfx862n from pulse to pulse with xcfx861nxe2x89xa0xcfx862n and xcfx861n greater than 0.
The latter step is important because it renders the down conversion process simple. The PSK technique guarantees unambiguous range measurement and the interleaved pulse sequences make the measurement robust against target maneuver. Before this background all important requirements can be fulfilled with the PSK waveform according to the invention.